High quality railgun HYPAC for hypervelocity impact experiments

The characteristic of a very high quality railgun HYPAC (10 kV, 6,000 μ F, 300 kJ) at the Institute of Space and Astronautical Science (ISAS) which is capable of accelerating a 1–2 gram projectile of 10–20 mm diameter made of polycarbonate to a hypervelocity of 7.8 km/sec and its utilization in hypervelocity impact experiments are reviewed. Also shown are further efforts to increase the velocity, to improve the quality of the projectile such as the acceleration of metal powder and to explore a new application of this facility.     

Crown-shaped stacked-capacitor cell for 1.5-V operation 64-Mb DRAMs

A self-aligned stacked-capacitor cell called the CROWN cell (a crown-shaped stacked-capacitor cell), used for experimental 64-Mb-DRAMs operated at 1.5 V, has been developed using 0.3-μm electron-beam lithography. This memory cell has an area of 1.28 μm². The word-line pitch and sense-amplifier pitch of this cell are 0.8 and 1.6 μm, respectively. In spite of this small cell area, the CROWN cell has a large capacitor surface area of 3.7 μm² because (1) it has a crown-shaped capacitor electrode, (2) its capacitor is on the data line, and (3) it has a self-aligned memory cell fabrication process and structure. The large capacitor area and a Ta₂O₅ film equivalent to a 2.8-nm SiO₂ film ensure a large storage charge of 33 fC (storage capacitance equals 44 fF) for 1.5-V operation. A small CROWN cell array and a memory test circuit were successfully used to achieve a basic DRAM cell operation     

Nonlinear Techniques for the Improvement of Signal-to-Noise Ratio

We propose a novel circuit for noise rejection, which is composed mainly of a linear amplifier, a nonlinear amplifier, and a filter. The input signal is applied to a linear amplifier, a bandpass limiter, and an envelope detector followed by a low-pass filter. The output of the lowpass filter is multiplied by the output of the bandpass limiter. The difference between the output of the multiplier and linear amplifier is the output of the proposed circuit. It is then indicated that the proposed circuit rejects noise, of which amplitude is narrow-band relative to the desired signal or noise phase.     

Promising storage capacitor structures with thin Ta2O5 film for low-power high-density DRAMs

To ensure the required capacitance for low-power DRAMs (dynamic RAMs) beyond 4 Mb, three kinds of capacitor structures are proposed: (a) poly-Si/SiO₂/Ta₂O₅/SiO₂ /poly-Si or poly-Si/Si₃N₄/Ta₂O ₅/SiO₂/poly-Si (SIS), (b) W/Ta₂O₅ /SiO₂/poly-Si (MIS), and (c) W/Ta₂O₅ W (MIM). The investigation of time-dependent dielectric breakdown and leakage current characteristics indicates that capacitor dielectrics that have equivalent SiO₂ thicknesses of 5, 4, and 3 nm can be applied to 3.3-V operated 16-Mb DRAMs having stacked capacitor cells (STCs) by using SIS, MIS, and MIM structures, respectively, and that 3 and 1.5 nm can be applied to 1.5-V operated 64-Mb DRAMs having STCs by using MIS and MIM structures, respectively. This can be accomplished while maintaining a low enough leakage current for favorable refresh characteristics. In addition, all these capacitors show good heat endurance at 950°C for 30 min. Therefore, these capacitors allow the fabrication of low-power high-density DRAMs beyond 4 Mb using conventional fabrication processes at temperatures up to 950°C. Use of the SIS structure confirms the compatability of the fabrication process of a storage capacitor using Ta₂O₅ film and the conventional DRAM fabrication processes by successful application to the fabrication process of an experimental memory array with 1.5-μm×3.6-μm stacked-capacitor DRAM cells     

Studies of the mechanism of boron nitride thin-film formation by means of nitrogen-argon gas mixture plasma jet

The mechanism of boron nitride thin-film formation using the nitrogen-argon gas mixture plasma jet, which is a new method reported in our previous paper, has been studied. The electron temperature, electron density, and magnitudes of various ion and radical species in the plasma jet have been measured by an electrostatic probe and by optical spectroscopic methods. The amount of recombination of free nitrogen atoms by three-body conversion was estimated using the emission intensity of the first positive band system from nitrogen B state vibrational level v = 11. The emission intensity from vibrational level v = 11 in the vicinity of the boron target shows a more than one order of magnitude increase than when the boron target is absent, and the peak value occurs at a gas pressure of approximately 5 Torr to 10 Torr. Experimental data are discussed in connection with the optimum discharge condition of BN synthesis in the plasma. It is concluded that enhanced recombination on the boron target and substrate surface supplies large amounts of molecular nitrogen ions and vibrationally excited nitrogen molecules in the vicinity of the substrate, and thus that the high-energy ion-rich condition makes it possible to form a cubic BN film on the substrate surface placed in the plasma jet. © 1997 Scripta Technica, Inc. Electr Eng Jpn, 120(4): 1–6, 1997     

Oxygen radical density measurement in O2–N2 gas mixture plasma by means of a thin platinum wire

The oxygen radical density was measured in a weakly ionized plasma in an O₂–N₂ gas mixture by using a simple platinum thin‐wire sensor. The increased temperatures of the platinum wire caused by the energy released in the recombination of two oxygen radicals on the platinum surface were measured by varying the nitrogen gas mixture ratio and were compared with the oxygen radical densities theoretically calculated under the same plasma conditions. The relation between the wire temperature and the oxygen radical densities was cross‐checked by quantitative measurement of oxygen radical densities with a quadrupole mass spectrometer. All of these results made it possible to determine the oxygen radical density experimentally from the platinum wire temperature alone. The influence of the ambient gas temperature on the radical density measurement is also discussed. © 2004 Wiley Periodicals, Inc. Electr Eng Jpn, 149(4): 14–20, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20018     

Improved Long-Term DC Drift in OH-Reduced Lithium Niobate Optical Intensity Modulators

Long-term dc drift of lithium niobate optical intensity modulators is successfully reduced by exclusion of OH(-)ions from the lithium niobate substrate. In this regard, the titanium-indiffused waveguide is prepared on a commercial OH-free substrate under a dry gas atmosphere.The dc drift measured for these hermetically sealed modulators indicates a possibility for continuous operation over 20 years at 50°C.     

Two-step annealing technique for leakage current reduction inchemical-vapor-deposited Ta2O5 film

A capacitor technology developed to obtain extremely thin Ta₂ O₅ dielectric film with an effective SiO₂ film thickness down to 3 nm (equivalent to 11 fF/μm²) for a 1.5-V, low-power, high-density, 64-Mb DRAM is discussed. The Ta₂ O₅ has low leakage current, low defect density, and excellent step coverage. The key process is two-step annealing after the deposition of the film by thermal chemical vapor deposition (CVD). The first step involves ozone (O₃) annealing with ultraviolet light irradiation, which reduces the leakage current. The second step is dry oxygen (O₂) annealing, which decreases the defect density. A more significant reduction in the leakage current is attained by the combination of the two annealing steps